The sealing of the ends of the heat treatment tunnels of conveyor tunnel furnaces has long presented a problem. This is especially true of furnaces whose chambers, or tunnels, must be constructed air tight for reasons of atmospheric oxidation prevention, partial pressure control of (explosive) gases or toxic gas containment.
Each of these applications requires atmospheric isolation yet the material being processed, whether it be continuous in the form of strips and/or wire, or bulk products riding on a conveyor belt or push boats, must have access to and from the furnace chamber. The means of access to and from the chamber ideally prevents any gases from crossing the boundary of the heating chamber of the furnace, while allowing the product to be treated to do so as easily as possible.
It has heretofore been proposed to provide access to the heating chamber through mechanical, or physical type seals.
Exemplary of such seals are the felt pad graphite block seals at each opposite end of the furnace in U.S. Pat. No. 2,057,518 of Oct. 13, 1936 to Fraser. A non-aqueous liquid seal using oil as the sealant is also disclosed in this patent. In U.S. Pat. No. 3,169,157 to Harris of Feb. 9, 1965 liquid sealing means in the form of opposed sponge pads saturated with water is disclosed.
U.S. Pat. No. 3,396,951 of Aug. 13, 1968 to Cope, reissued as No. Re: 28,168 of Sept. 24, 1974, not only discloses upper and lower nip rolls for sealing the furnace against admission of air, but also discloses a gate slidable over an opening in a wall for maintaining pressure and the immersion of a treated wire in a molten zinc bath at the end of the treatment zone.
Single pivoted gates, like single slidable gates are also old in the art as exemplified in U.S. Pat. No. 2,942,742 to Wilbur of June 28, 1960 wherein single gates between chambers are automatically actuated by limit switches which are engaged by trays on the conveyor.
Single article actuated entrance and exit radiation shield gates or doors are disclosed in U.S. Pat. No. 3,130,250 to Mescher of Apr. 21, 1964.
Pressurized sealing tunnels having multiple, article-actuated, gates, or flaps, to maintain a seal pressure less than furnace pressure, or to retain furnace gas are disclosed in U.S. Pat. No. 2,253,897 to Doderer of Aug. 26, 1941, U.S. Pat. No. 3,467,366 to Westeren, et al, of Sept. 26, 1969 and U.S. Pat. No. 3,606,288 to Bloom of Sept. 20, 1971.
However, such seals are gas retarders and not positive, fully effective gas seals making it impossible for expensive treatment gas to leak out or damaging air to leak into the furnace.